The present invention relates to an improved process for the manufacture of disodium phosphate. More particularly the invention relates to an improved process for the manufacture of hydrated disodium phosphate of a high purity, which does not involve the use of organic solvents.
Disodium phosphate is an important industrial chemical used in detergent industry and other products. It is normally made by reacting H.sub.3 PO.sub.4 with alkali metal hydroxide or carbonate. The naturally occurring ring alkali metals, such as sodium or potassium, are most commonly mined and recovered as a salt, such as chloride. Thus in order to create phosphoric acid with an alkali metal hydroxide or carbonate, the alkali metal salt must first be converted to the corresponding hydroxide or carbonate. It is thus readily apparent that a process for preparing disodium phosphate which employs NaCl directly, eliminating expensive intermediate processing, would be a highly desirable and economically attractive process.
In principle, the primary reaction between sodium chloride and phosphoric acid may result in a sodium phosphate salt, such as NaH.sub.2 PO.sub.4 and hydrochloric acid. The practice of this reaction by heating the two reagents, will result in the expulsion of hydrogen chloride, but at the same time would cause a dehydration of the orthophosphate to form the metaphosphate salt (NaPO.sub.3). Furthermore, this system is extremely corrosive at the temperatures necessary to expel the hydrogen chloride. The corrosion problem is quite severe and can not be readily overcome and will also result the incorporation of various impurities in the product.
In order to obviate the corrosion problem, some processes were suggested in the last twenty years, in which organic solvents are utilized for removing the hydrogen chloride and thus temperatures in the range of 40 to 100 degrees centigrade may be utilized. Thus according to U.S. Pat. No. 3,443,889 a method is described for the manufacture of disodium phosphate, from sodium chloride and phosphoric acid in the presence of a water immiscible primary alkyl amine containing from 8 to 22 carbon atoms and a water immiscible liquid organic solvent. The organic phase containing the hydrogen chloride is separated from the reaction mixture, the amine being regenerated by treating the organic liquid with an alkaline reagent and its recovery from the remainder reaction mixture. Although the process seems to be quite interesting from a theoretical point of view, in practice its exploitation is quite doubtfully, in view of the expensive solvents required and losses thereof which are inherent during the extraction and regeneration. It should be borne in mind that large amounts of solvents are required in the process, these solvents having molecular weights in the range of 180 to 315, and even small losses of solvent will increase appreciable the overall costs of the process. According to U.S. Pat. No. 3,661,513 sodium phosphate having the formula Na.sub.1.5 H.sub.1.5 PO.sub.4 is obtained by reacting sodium chloride with phosphoric acid in the presence of a secondary or tertiary amine containing from 6 to 24 carbon atoms. The steps are quite similar to those of the previous patent and has the same disadvantages.
In order to complete the state of the art in respect to the manufacture of sodium phosphate, it should perhaps be interesting to mention its preparation from sodium chloride and phosphoric acid (as described in Israeli Pat. No. 9539) carried out at ambient temperature. According to this process, a great excess of phosphoric acid is utilized, and the mixture of phosphoric acid and hydrochloric acid is removed by solvent extraction using water immiscible organic solvents. The sodium phosphate salt is crystallized from the aqueous solution resulted after removal of the acids. The process is indeed quite elegant, but its implementation on a commercial scale is questionable particularly in view of the high costs involved concerning the organic solvents and energy required to separate the phosphoric acid from hydrochloric acid.